Multicolor fluorescence nanoscopy by photobleaching: concept, verification, and its application to resolve selective storage of proteins in platelets.

Fluorescence nanoscopy provides means to discern the finer details of protein localization and interaction in cells by offering an order of magnitude higher resolution than conventional optical imaging techniques. However, these super resolution techniques put higher demands on the optical system and the fluorescent probes, making multicolor fluorescence nanoscopy a challenging task. Here we present a new and simple procedure, which exploits the photostability and excitation spectra of dyes to increase the number of simultaneous recordable targets in STED nanoscopy. We use this procedure to demonstrate four-color STED imaging of platelets with ≤40 nm resolution and low crosstalk. Platelets can selectively store, sequester, and release a multitude of different proteins, in a manner specific for different physiological and disease states. By applying multicolor nanoscopy to study platelets, we can achieve spatial mapping of the protein organization with a high resolution for multiple proteins at the same time and in the same cell. This provides a means to identify specific platelet activation states for diagnostic purposes and to understand the underlying protein storage and release mechanisms. We studied the organization of the pro- and antiangiogenic proteins VEGF and PF-4, together with fibrinogen and filamentous actin, and found distinct features in their respective protein localization. Further, colocalization analysis revealed only minor overlap between the proteins VEGF and PF-4 indicating that they have separate storage and release mechanisms, corresponding well with their opposite roles as pro- and antiangiogenic proteins, respectively.